The invention relates to a power supply system comprising: an apparatus with a rechargeable battery, and a power supply unit with a switched-mode power supply, which power supply unit can be coupled to the apparatus to supply current to the apparatus.
The invention also relates to a power supply unit and to an apparatus with a rechargeable battery for such a power supply system.
Such a power supply system is known, inter alia, from U.S. Pat. No. 4,775,827. In this known power supply system the power supply unit comprises a DC-DC converter, which converts the rectified mains voltage into a low direct voltage with which the battery of the apparatus is charged. In order to obtain greater design freedom there is a trend to provide a physical separation between the power supply unit and the apparatus, the power supply unit being constructed as an adapter which can be plugged into the wall outlet of the mains and the rechargeable apparatus being coupled to the adapter by a cord. Moreover, it is desirable that the power supply unit can rapidly charge the battery of the apparatus. However, this involves the risk that the battery is overloaded and becomes defective In order to preclude this, the apparatus should be capable in some way or another of communicating to the power supply unit that the battery is full and that charging should be stopped and should be continued with a safe small charging current, also referred to as trickle charging current. This communication capability requires a communication channel between the apparatus and the power supply unit. This communication channel requires additional wires in the connecting cord.